In many applications in the oil and gas industry it is desirable to transmit fluid pressure to a remote location for actuation of equipment, as well as to run electrical or other types of conductors for either transmission of signals or power to or from the surface to a subsurface location or for other reasons. Typically, a conduit which, if small and sufficiently flexible, can be unrolled from a roll is run along side the production tubing or otherwise into a borehole. If signals are to be sent from the wellbore to the surface electrically, a separate cable has been used, which many times is bundled to the exterior of the control tubing such that the hydraulic signals pass through the control tubing while the electrical, generally low-voltage signals, which record any number of downhole well conditions or operate low-voltage equipment, use the adjacent cable for transmission of such signals. It has also been attempted in the past to run the electrical signal cable into and through a coiled tubing unit. In those instances, the signal cable is externally shielded to prevent any signal interference from the surrounding tubing structure. One of the problems in this type of installation has been that the shielded cable would develop flaws or pinholes in its outer protective casing, which would then allow the fluids to migrate into the cable, damaging the signal conductors therein. Additionally, another problem encountered with such designs is that the conductor cable running through the tubing could in many places orient itself adjacent the tubing wall, particularly if the well was in any way deviated. The contact between the electrical cable and the tubing wall could cause two problems. First, it could cause abrasion of the shield material against the inside surface of the tubing wall, which ultimately would result in compromising the integrity of the covering for the conductors. This, as previously described, could cause a breakdown in the ability to transmit signals through the conductors. Additionally, close proximity to the tubing wall also rendered the internal cable vulnerable to damage from mechanical impacts on the tubing in situations where the cable is located up against the inside tubing wall. Such impacts could cause dents in the tubing wall, which would translate directly to the cable damaging and perhaps severing the cable. Finally, and to a lesser extent, close proximity to the inside wall of the tubing also created some potential risk of signal interference from the metallic tubing wall.
Space is routinely at a premium in oil and gas installations, particularly in offshore applications. It is frequently desirable that the external control tubing have a small diameter as possible, while, at the same time, it must have the necessary rigidity and internal diameter to allow accommodation of an internal conductor. What is desirable and heretofore lacking in the known equipment is a compact design where a conductor can be effectively isolated and located reasonably centrally to the tubing to minimize damage to the cable from impacts to the tubing. Additionally, with the conductors positioned within the tubing and their position retained away from the tubing wall, the spaces around the conductor can be used to allow fluid flow or, in the alternative, can be filled with a sealing material which provides further durability to the assembly of conductors, insulators/centralizers, and void sealant, all disposed within the tubing.